It is desired to produce accurate and reliable encoders that are made to operate in a secure manner so as to allow their basic function to be achieved even if certain elements of which they are composed develop a fault.
An optical angular encoder typically consists of a disc bearing regular marks, this disc being caused to rotate by actuation of a control button (for example a manual control button). A photoelectric cell fixed in front of the disc detects the filing past of the successive marks when the control button rotates the disc. The marks are typically openings in an opaque disc, a light-emitting diode being placed on one side of the disc and the photoelectric cell being placed on the other side.
Each passage of a mark constitutes an increment of a unit in the counting of the rotation of the disc. The angular resolution is determined by the angular pitch of the marks regularly disposed over a disc revolution. To detect both increments and decrements of angle of rotation when the direction of rotation is reversed, two photoelectric cells are provided, physically offset by an odd number of quarters of a pitch between them. Thus, the lit/unlit logic states of the two cells are encoded on two bits which successively take the following four successive values 00, 01, 11, 10 when the disc rotates in one direction and the following four successive values 00, 10, 11, 01 when the disc rotates in the other, so that it is easy to determine, not only the appearance of an increment of rotation (change of state of one of the bits) but the direction of the increment (by comparison between a state of the cells and the immediately prior state).
To increase the reliability of systems using such encoders, in particular for aeronautical applications, it has been proposed that the encoder be dualized or at least that the photoelectric cells inside the encoder be dualized. This makes it possible in part to detect faults such as the nonoperation of a light-emitting diode or of a detection diode since the states provided by the two cells are compared and the incrementation or decrementation information is validated only if it is provided in an identical manner by both encoders or both groups of photoelectric cells of the encoder. If the information is not identical it may be concluded that one cell at least (light-emitting diode or detection diode) is faulty and the count is invalidated by giving an error or fault indication signal.
However, this comparison of the signals of the two groups of cells turns out to be difficult since the positioning of the first group of cells must be rigorously identical to the positioning of the second group: while the offset by an odd number of quarters of a pitch of the marks between the two cells of one and the same group may be slightly inaccurate, the offset between the two groups of cells must be very accurately a multiple of the pitch spacing of the marks.
If matters are not so, it leads to a situation in which the incrementations or decrementations determined by the cells do not occur rigorously at the same moment. Admittedly the increments and decrements will be detected by the two groups of cells, but perhaps with a very slight offset in time. Consequently, it becomes possible for the computer, responsible for polling the counters associated with the two groups of cells in order to monitor the consistency of the indications given by these counters, to find at a given moment that the indications are not identical whereas, if it had polled a very short instant afterwards, it would have found identical indications.
To solve this problem, provision may be made for a delay in validating the comparison, that is to say the computer provides an error indication only if this error persists for a certain time. However the time for which it is necessary to wait is poorly determined since it depends on the speed of rotation of the button. For a manual control, the time that it is necessary to wait will be longer if the user turns the button more slowly. This leads to the fault indication being postponed for example by two seconds, this not always being acceptable. Moreover, this procedure consists in seeing faults since the computer detects them as potential faults but in regarding them as false faults for a certain time even if they are true faults. This solution is not satisfactory.